Photoinduced Generation Research Articles

A Well-Coupled Supramolecular System Accelerates Photophosphorylation.

Supramolecular assembly allows multiple chemical/bio-components integrated as one system for cascade biochemical reactions. Herein the graphitic carbon nitrides (g-C3N4) as photocatalyst trapped in a dipeptide hydrogel covering adenosine triphosphate (ATP) synthase accelerates the photophosphorylation through ATP synthesis. Self-assembled N-fluorenylmethoxycarbonyl diphenylalanine (Fmoc-FF) as nanofibrils to allow g-C3N4 nanosheets are embedded as a complex Fmoc-FF/g-C3N4 hydrogel. Fmoc-FF gel exhibits good electronic coupling with g-C3N4, which enables a photo-induced proton generation. The transmembrane proton gradient can be established by ATP synthase-lipid reconstituted on the surface of the Fmoc-FF/g-C3N4 hydrogel to enhance the ATP synthesis. It indicates that the Fmoc-FF/g-C3N4/ATP synthase-lipid film can possess a longer-term ATP production capability and allow repeated immersion for sustained ATP production. Such a hydrogel-supported ATP synthesis platform is achieved by a procedure at a larger scale.

A high-performance selenium nanoflake-based avalanche photodetector

Photodetectors are now indispensable in our daily lives, and there is a pressing need to explore new materials and mechanisms that can push the boundaries of device performance. Two-dimensional (2D) van der Waals (vdW) semiconductors have emerged recently as a promising material platform with exceptional optoelectronic properties, making them particularly suitable for high-performance photodetectors. However, photoinduced carrier generation in conventional 2D vdW photodetectors are usually limited, and new mechanisms need to be introduced to enhance device performance. Herein, we report a high-performance avalanche photodetector based on selenium (Se) nanoflakes. Our device achieves a high photoresponsivity (R) and specific detectivity (D*) of 361 A·W−1 and 2.4 × 1012 Jones, respectively. These figures of merit are two orders of magnitude higher than that in conventional Se photoconductive photodetectors. As a large bandgap vdW semiconductor, the Se channel allows the application of an extremely large bias voltage across it, and the resulting high electric field leads to the avalanche multiplication of carriers, which lays the groundwork for the improved device performance.

Dynamic, multi-color, multi-level stimulus-responsive luminescence in a co-crystallized coordination polymer: Modulation of energy transfer by acid-induced ligand protonation and photo-induced radical generation

Stimulus-responsive luminescent materials (SLMs), which can respond to external stimuli by switching their luminescence properties, hold great promise for a variety of applications. Particularly, multi-level SLMs with a logical stimulus sequence and a dynamic, multi-color transition are highly desirable for anti-counterfeiting and information encryption, but an effective design strategy is lacking. Herein, a co-crystallized coordination polymer multi-level SLM [Cd(9-AC)4]·(HAD)2·(9-HAC)2 (9-HAC for anthracene-9-carboxylic acid and AD for acridine) with dual emission centers of coordination unit ([Cd(9-AC)4]2-) and exciplex ([9-HAC/HAD]+) has been successfully constructed by a stepwise solvent process. This SLM possesses a variety of stimulus-responsive properties, including mechanochromic (MC), mechanochromic luminescence (MCL), reversible acidochromic luminescence (ACL), and reversible photochromic luminescence (PCL). Notably, both ACL and PCL properties are achieved by modulating the energy transfer process between the coordination unit and the exciplex, while there is a logical sequence between these two stimulus actions. The acid-induced activation and radical-regulated PCL performance has been exploited in the development of anti-counterfeiting and information encryption applications. This work provides an effective strategy to realize dynamic, multi-color, multi-level stimuli-responsive luminescence by modulating the energy transfer through acid-induced ligand protonation and photo-induced radical generation.

Multimodal integrated and broadband light-driven antibacterial cellulose fabric based on π-π coupling enhanced intermolecular FRET

Fabrication of antimicrobial photodynamic therapy (aPDT) materials based on organic photosensitizers has garnered considerable attention within functional textiles. However, the UV- or narrow-band absorption range of the photosensitizers results in poor photon utilization of the fabrics, limiting the photodynamic efficiency and wasting solar energy. In this study, a broadband light-driven antibacterial cellulose fabric (CF-ZnPc/NAD) was developed by loading carboxyl-modified zinc(II) phthalocyanine photosensitizer (CAZnPc) and cationic 1,8-naphthalimide fluorescent molecule (NAD) on the fabric via covalent binding and electrostatic adsorption assembly, facilitating the intermolecular π-π coupling and fluorescence resonance energy transfer (FRET) process. There is a 2.54-fold increase in photo-induced ROS generation capacity of CF-ZnPc/NAD via the FRET process compared to that of CF-ZnPc, and it also exhibited a strong photothermal effect (PTT), wherein the temperature of the fabric increased from 24.5 to 53.5 °C within 80 s of illumination (λ > 400 nm, 75 mW/cm2). CF-ZnPc/NAD exhibited strong light-harvesting capacity and a combination of aPDT and PTT, achieving excellent antibacterial performance against Staphylococcus aureus (Gram-positive, S. aureus) and Escherichia coli (Gram-negative, E.coli) with 99.99 % bacterial reduction under 90 min of illumination (λ > 400 nm, 10 ± 1 mW/cm2). This study demonstrates a novel and facile strategy for successfully fabricating high-performance antibacterial cellulose fabrics with potential biomedical prospects.

Regulation of organic solar cells performance through external electric field: From charge transfer mechanisms to photovoltaic properties

In organic solar cells (OSCs), comprehending the charge transfer mechanism at D/A interfaces is crucial for photoinduced charge generation and enhancing power conversion efficiency (PCE). The charge transfer mechanism and photovoltaic performance of the parallel stacking interface configuration of the PTQ10 polymer donor and T2EH non-fullerene acceptor (NFA) are systematically studied at the microscopic scale. The analysis of the electron-hole distribution of the PTQ10/T2EH excited states revealed the presence of multiple charge excitation modes and charge transfer pathways. Using Marcus theory, we examine the charge separation rate (KCS) of PTQ10/T2EH under external electric field (Fext) modulation, and it is clarified that reorganization energy (λ) is the main factor that affects the KCS. Our results show that Fext has a positive impact on the photovoltaic properties of PTQ10/T2EH thin films, as evidenced by the modulation of the open circuit voltage (VOC), voltage loss (VLOSS) and fill factor (FF). Overall, this study provides valuable theoretical insights for Fext to accelerate the charge separation process and enhance photovoltaic efficiency.

Heterophase structure of ZnSnO3 (rhombohedral and orthorhombic) for efficient dye degradation and N2-to-NH3 conversion via piezocatalysis and piezo-photocatalysis

Fabricating heterophase junctions proves to be an efficient strategy for achieving efficient charge separation and enhancing the catalytic activity of photocatalysts. Surprisingly, this approach is rarely utilized in the field of piezocatalysis. In this work, a novel ZnSnO3 heterophase junction catalyst was fabricated by combining rhombohedral (R-ZnSnO3) and orthorhombic (O-ZnSnO3) phases for efficient piezo and piezo-photocatalytic RhB degradation and N2 fixation. The piezocatalytic RhB degradation rates of ZnSnO3 heterophase catalysts are 5.6 and 3.6 times higher than those of O-ZnSnO3 and R-ZnSnO3, respectively, with similar enhancements observed in piezocatalytic N2 fixation. The coupling of R-ZnSnO3 and O-ZnSnO3 does not increase the piezoelectric properties; however, an S-scheme structure formed at the interface. Under the combined effects of band bending, interface electric fields, and Coulomb attraction, electrons and holes migrate and achieve effective spatial separation. This result supports the energy band theory over the induction charge theory, contributing to the ongoing debate in piezocatalytic mechanisms. Electrochemical analyses and piezo-photocatalytic tests further indicate that ZnSnO3 heterophase junctions exhibit a synergistic effect between piezocatalysis and photocatalysis. This synergy can be attributed to the complementary effects of photoinduced carrier generation and piezoelectric field-assisted carrier separation. Hydroxyl radicals, superoxide radicals, and holes serve as the primary active species responsible for piezocatalytic RhB degradation through a mechanism similar to photocatalysis. Theoretical calculations reveal that the Sn site on the surface of O-ZnSnO3 acts as the active site within the ZnSnO3 heterophase junction, providing valuable insights for future catalyst development. This work offers a deeper understanding of the synergistic mechanisms in piezo-photocatalysis, presenting a promising route for efficient environmental and energy applications.

Visible Light Induced B-H Bond Insertion Reaction with Diazo Compounds.

A protocol induced by visible light for the direct insertion of α-carbonyl carbenes into the B-H bond of amine-borane adducts has been developed under conditions that are free of metal and photocatalyst. This approach provides a straightforward route to various organoboron compounds from diazo compounds and amine-borane adducts with moderate to good yields. Mechanistic investigations reveal that this photoinduced reaction proceeds through concerted carbene insertion into the B-H bond, and the photoinduced generation of free carbene from α-diazo esters may be the rate-determining step.

Different Reaction Mechanisms Triggered by the Meta Effect: Photoinduced Generation of Quinone Methides from Hydroxybiphenyl Derivatives.

A series of sterically congested quinone methides (QMs) exhibit photoinduced antiproliferative activity against some human cancer cell lines. To elucidate the structure-reactivity relationship and details of mechanisms of the photogeneration of sterically congested QMs, we chose phenylphenol derivatives 1-3 as QM precursors and investigated their photodehydration processes in aqueous solutions using ultrafast spectroscopy and theoretical computations. We found that meta derivatives 1 and 2 undergo water-mediated excited-state proton transfer (ESPT) from the phenol OH, followed by expulsion of the OH- to form QMs. By comparison, para derivative 3 proceeds via water-mediated ESPT from H2O to benzyl alcohol coupled with dehydration as the first step, delivering a cation intermediate, which further deprotonates to yield QM. Such results would help chemists understand more about the meta effects in photochemistry and about ESPT and would help synthetic chemists design sterically congested QM precursors with extraordinary reactivities and expand applications of QMs in biological and medical systems.

(Invited) Impact of Anchors on the Photophysical Processes in Water-Soluble Fullerene Conjugates

Fullerenes are excellent acceptors in both photoinduced electron- and energy-transfer reactions. Many studies on the photophysical processes of fullerenes have been reported in the last decades, especially aiming towards the development of electronic devices. However, only a few studies have been reported on biocompatible fullerene systems, mainly due to the extremely low solubility of C60 in both water and water-miscible solvents. In our previous studies, we reported the synthesis of fullerene-PEG conjugates and demonstrated their ability for photoinduced DNA damage and ROS generation upon visible light irradiation.2 In this work, we investigated the effect of the water-soluble anchors on the photophysical properties of fullerene molecules. The generation of ROSs in water-soluble fullerene-peptide conjugates were tested by electron paramagnetic resonance (EPR) methods in comparison to previously reported fullerene-PEG conjugates above. To better understand the mechanism in ROS generation, a comprehensive investigation was performed by transient absorption spectroscopic data, encompassing the dynamics of photon absorption, exciton formation, and charge transfer mechanisms. Additionally, insights derived from density functional theory (DFT) calculations and in vitro assessments of photocytotoxicity will be discussed in detail. Figure 1

Threonine Aldolase-Catalyzed Enantioselective α-Alkylation of Amino Acids through Unconventional Photoinduced Radical Initiation.

Visible light-driven pyridoxal radical biocatalysis has emerged as a promising strategy for the stereoselective synthesis of valuable noncanonical amino acids (ncAAs). Previously, the use of well-tailored photoredox catalysts represented the key to enable efficient pyridoxal phosphate (PLP) enzyme-catalyzed radical reactions. Here, we report a PLP-dependent threonine aldolase-catalyzed asymmetric α-C-H alkylation of abundant amino acids using Katritzky pyridinium salts as alkylating agents. The use of engineered threonine aldolases allowed for this redox-neutral radical alkylation to proceed efficiently, giving rise to challenging α-trisubstituted and -tetrasubstituted ncAA products in a protecting-group-free fashion with excellent enantiocontrol. Mechanistically, this enantioselective α-alkylation capitalizes on the unique reactivity of the persistent enzymatic quinonoid intermediate derived from the PLP cofactor and the amino acid substrate to allow for novel radical C-C coupling. Surprisingly, this photobiocatalytic process does not require the use of well-established photoredox catalysts and operates through an unconventional photoinduced radical generation involving a PLP-derived aldimine. The ability to develop photobiocatalytic reactions without relying on classic photocatalysts or photoenzymes opens up new avenues for advancing stereoselective intermolecular radical reactions that are not known in either organic chemistry or enzymology.

Titanium Superoxide as a Carrier of a "Long-Lived" Superoxide Anion: An Ab Initio Investigation.

The photoinduced generation of a superoxide anion on the surface of a semiconductor photocatalyst is usually attributed to the reduction of O2 with conduction-band electrons. In the current work, the reaction of TiO2 with O2 giving rise to TiO4 in superoxide and peroxide states has been investigated with ab initio (CAS, CCSD) and DFT (B3LYP) calculations. The ground triplet state and two substates (open-shell singlet (OSS) and closed-shell singlet (CSS)) of a doubly degenerate excited singlet state (a1Δg) are considered as reactive states of oxygen, participating in spontaneous or photoinduced processes, respectively. The triplet and OSS singlet states of TiO4 contain O2- as structural units and can be defined as titanium superoxides. Both states have energy less than the level of the initial pair TiO2+O2 by about 30 kcal/mol. The CSS state of TiO4 has a diperoxide structure Ti4+(O22-)2 and also lies in energy below the initial pair TiO2+3O2. Titanium superoxide is considered to be the carrier of an "exceptionally stable" and "long-lived" superoxide anion, which was earlier synthesized or detected on the surface of TiO2. The low-energy location of the conical intersections on the way from reagents to 3TiO4 allows us to explain the literature data on the spontaneous generation of the "long-lived" superoxide anion on the TiO2 surface.

Photo-Induced Generation of Oxygenated Quaternary Centers via EnT Enabled Singlet O2 Addition to C3-Maleimidated Quinoxaline: A Reagent-Less Approach.

Demonstrated here is an external photo-sensitizer-free (auto-sensitized) singlet oxygen-enabled solvent-dependent tertiary hydroxylation and aryl-alkyl spiro-etherification of C3-maleimidated quinoxalines. Such "reagent-less" photo-oxygenation at Csp3-H and etherification involving Csp3-H/Csp2-H are unparalleled. Possibly, the highly π-conjugated N-H tautomer allows the substrate to get excited by irradiation, and subsequently, it attains the triplet state via ISC. This excited triplet-state sensitized molecule then transfers its energy to a triplet-state oxygen (3O2) generating reactive singlet oxygen (1O2) for hydroxylation and spirocyclization depending on the solvent used. In HFIP, the generated alkoxy radical accepts a proton via HAT giving hydroxylated product. In contrast, in an aprotic PhCl it underwent a radical addition at the ortho-position of the C2 aryl to provide spiro-ether. An unprecedented orthogonal spiro-etherification was observed via the displacement of o-substitutents for ortho (-OEt, -OMe, -F, -Cl, -Br) substituted substrates. The order of ipso substitution follows the trend -OMe>-OEt>-F>-H>-Cl>-Br. Both these oxygenation reactions can be carried out with nearly equal ease using direct sunlight without the requirement of any elaborate reaction setup. Demonstration of large-scale synthesis and a few interesting transformations have also been realized. Furthermore, several insightful control experiments and quantum chemical computations were performed to unravel the mechanism.

Full phonon dispersion along the stacking direction in nanoscale van der Waals materials by picosecond acoustics

Phonon dispersion in crystals determines many important material properties, but its measurement usually requires large-scale facilities and is limited to bulk samples. Here, we demonstrate the measurement of full phonon dispersion along the stacking direction in nanoscale systems by using picosecond acoustics. A heterostructure sample was prepared consisting of layers of hexagonal boron nitride (hBN) sandwiching a thin layer of black phosphorus (BP), within which a strain pulse was generated by photoexcitation and observed with an optical probe in the BP layer. The strain pulse traverses to the few nanometer thick hBN layers, where it propagates to the edge and echoes back, like acoustic waves in Newton’s cradle. The echoes returning to the BP layer provide information on the frequency-dependent time-of-flight and group velocity dispersion of the sample system. The microscopic origin of the photoinduced strain pulse generation and its propagation is revealed from first principles. Phonon frequency combs observed in the Fourier transform spectrum confirm the strain wave round trips and demonstrate the feasibility of determining group velocity dispersion through photoacoustics.

Ultrafast Terahertz Superconductor Van der Waals Metamaterial Photonic Switch

The high‐temperature layered superconductor (HTS) BSCCO is one of the key quantum material platforms in THz science and technology. Compact, stable, and reliable BSCCO THz photonic integrated circuit components can be developed to effectively and efficiently control and manipulate THz wave radiation, especially for future communication systems and network applications. Herein, the design, simulation, and modeling of ultrafast THz metamaterial photonic integrated circuits are reported on a few nanometer‐thick HTS BSCCO van der Waals (vdWs), capable of the active modulation of phase with constant transmission coefficient over a narrow‐frequency range. Meanwhile, the metamaterial circuit works as an amplitude modulator without significantly changing the phase in a different frequency band. Under the application of ultrashort optical pulses, the transient modulation dynamics of the THz metamaterial offer a fast‐switching timescale of 50 ps. The dynamics of picosecond light–matter interaction, Cooper pairs breaking, photoinduced quasiparticles generation and recombination, phonon bottleneck effect, and emission and relaxation of bosons in BSCCO vdW metamaterial arrays are discussed for the potential application of multifunctional superconducting photonic integrated circuits in communication and quantum technologies.

Generation and Application of Aryl Radicals Under Photoinduced Conditions.

Photoinduced aryl radical generation is a powerful strategy in organic synthesis that facilitates the formation of diverse carbon-carbon and carbon-heteroatom bonds. The synthetic applications of photoinduced aryl radical formation in the synthesis of complex organic compounds, including natural products, physiologically significant molecules, and functional materials, have received immense attention. An overview of current developments in photoinduced aryl radical production methods and their uses in organic synthesis is given in this article. A generalized idea of how to choose the reagents and approach for the generation of aryl radicals is described, along with photoinduced techniques and associated mechanistic insights. Overall, this article offers a critical assessment of the mechanistic results as well as the selection of reaction parameters for specific reagents in the context of radical cascades, cross-coupling reactions, aryl radical functionalization, and selective C-H functionalization of aryl substrates.

Synthesis and characterization of water-soluble C60-peptide conjugates.

With the aim of developing biocompatible and water-soluble C60 derivatives, three types of C60-peptide conjugates consisting of hydrophilic oligopeptide anchors (oligo-Lys, oligo-Glu, and oligo-Arg) were synthesized. A previously reported Prato reaction adduct of a biscarboxylic acid-substituted C60 derivative was subjected to a solid phase synthesis for amide formation with N-terminal amines of peptides on resin to successfully provide C60-peptide conjugates with one C60 and two peptide anchors as water-soluble moieties. Among three C60-peptide conjugates prepared, C60-oligo-Lys was soluble in water at neutral pH, and C60-oligo-Glu was soluble in buffer with a higher pH value, but C60-oligo-Arg was insoluble in water and most other solvents. C60-oligo-Lys and C60-oligo-Glu were characterized by 1H and 13C NMR. Photoinduced 1O2 generation was observed in the most soluble C60-oligo-Lys conjugate under visible light irradiation (527 nm) to show the potential of this highly water-soluble molecule in biological systems, for example, as a photosensitizer in photodynamic therapy.

Nitric Oxide Photorelease from Silicone Films Doped with N-Nitroso BODIPY.

Nitric oxide (NO) is a unique biochemical mediator involved in the regulation of vital processes. Light-controllable NO releasers show promise in the development of smart therapies. Here, we present a novel biocompatible material based on polydimethylsiloxane (PDMS) doped with BODIPY derivatives containing an N-nitroso moiety that is capable of the photoinduced generation of NO. We study the green-light-induced NO-release properties with the following three methods: electrochemical gas-phase sensor, liquid-phase sensor, and the Griess assay. Prolonged release of NO from the polymer films after short irradiation by narrow-band LED light sources and a laser beam is demonstrated. Importantly, this was accompanied by no or little release of the parent compound (BODIPY-based photodonor). Silicone films with the capability of controllable and clean NO release can potentially be used as a highly portable NO delivery system for different therapeutic applications.

Photoinduced charge generation of nanostructured carbon derived from human hair biowaste for performance enhancement in polyvinylidene fluoride based triboelectric nanogenerator

Carbon nanostructures derived from human hair biowaste are incorporated into polyvinylidene fluoride (PVDF) polymer to enhance the energy conversion performance of a triboelectric nanogenerator (TENG). The PVDF filled with activated carbon nanomaterial from human hair (AC-HH) exhibits improved surface charge density and photoinduced charge generation. These remarkable properties are attributed to the presence of graphene-like nanostructures in AC-HH, contributing to the augmented performance of PVDF@AC-HH TENG. The correlation of surface morphologies, surface charge potential, charge capacitance properties, and TENG electrical output of the PVDF composites at various AC-HH loading is studied and discussed. Applications of the PVDF@AC-HH TENG as a power source for micro/nanoelectronics and a movement sensor for detecting finger gestures are also demonstrated. The photoresponse property of the fabricated TENG is demonstrated and analyzed in-depth. The analysis indicates that the photoinduced charge carriers originate from the conductive reduced graphene oxide (rGO), contributing to the enhanced surface charge density of the PVDF composite film. This research introduces a novel approach to enhancing TENG performance through the utilization of carbon nanostructures derived from human biowaste. The findings of this work are crucial for the development of innovative energy-harvesting technology with multifunctionality, including power generation, motion detection, and photoresponse capabilities.

Ruthenium Pyrazole Complexes: A Family of Highly Active Metallodrugs for Photoactivated Chemotherapy.

Ruthenium complexes bearing bis pyrazole (pzH) ligands, cis-[Ru(bpy)2(R-pzH)2]2+ (bpy = 2,2'-bipyridine, R = -H, -Cl), were examined as photoactivated anticancer prodrugs. A dicationic pyrazole complex deprotonated to give monocationic pyrazole-pyrazolate complexes, cis-[Ru(bpy)2(R-pz-)(R-pzH)]+, in an aqueous solution with pKa values of 9.5 and 7.2 for R = H and R = Cl, respectively. Upon deprotonation, relative quantum yields of photosubstitution decreased while lipophilicity of the complexes increased according to the measurements of water-octanol coefficients. The ruthenium complex with 4-chloropyrazole ligands displayed high cytotoxicity upon light irradiation (IC50 = 0.060 ± 0.016 μM) toward lung cancer cells, which was 7 times higher than that in the dark (IC50 = 0.44 ± 0.07 μM). Additional experiments for the ruthenium R-pyrazole complexes indicated that (1) selective photodissociation of the 4-chloropyrazole ligand occurs from cis-[Ru(bpy)2(4-Clpz-)(4-ClpzH)]+, (2) photoinduced ligand dissociation is dominant rather than photoinduced generation of singlet oxygen (1O2), and (3) induction of cell death occurs via the intrinsic pathway of apoptosis.

A Green Environmental Protection Photocatalytic Molecular Reactor for Aerobic Oxidation of Sulfide to Sulfoxide.

The design and synthesis of metal-organic frameworks (MOFs) as photocatalytic molecular reactors for varied reactions have drawn great attention. In this work, we designed a novel photoactive perylenediimides-based (PDI) carboxylate ligand N,N'-di(3',3",5',5"-tetrakis(4-carboxyphenyl))-1,2,6,7-tetrachloroperylene-3,4,9,10-tetracarboxylic acid diimide (Cl-PDI-TA) and use it to successfully synthesize a novel Zr(IV)-based MOF 1 constructed from [Zr6 O8 (H2 O)8 ]8+ clusters bridged by Cl-PDI-TA ligands. Structural analysis revealed that Zr-MOF 1 manifests a 3D framework with (4,8)-connected csq topology and possesses triangular channels of ~17 Å and mesoporous hexagonal channels of ~26 Å along c-axis. Moreover, the synthesized Zr-MOF 1 exhibits visible-light absorption and efficient photoinduced free radical generation property, making it a promising photocatalytic molecular reactor. When Zr-MOF 1 was used as a photocatalyst for the aerobic oxidation of sulfides under irradiation of visible light, it could afford the corresponding sulfoxides with high yield and selectivity. Experimental results demonstrated that the substrate sulfides could be fixed in the pores of 1 and directly transformed to the products sulfoxides in the solid state. Furthermore, the mechanism for the photocatalytic transformation was also investigated and the results revealed that the singlet oxygen (1 O2 ) and superoxide radical (O2 ⋅- ) generated by the energy transfer and electron transfer from the photoexcited Zr-MOF to oxidants were the main active species for the catalytic reactions. This work offers a perceptive comprehension of the mechanism in PDI-based MOFs for further study on photocatalytic reactions.